The invention relates to a safety bearing for retaining a rotor shaft of a machine.
Magnetic bearings are increasingly used nowadays for mounting rotating rotor shafts of machines during operation, said magnetic bearings maintaining the rotating rotor shaft in a floating state with the aid of magnetic fields. In the event that the magnetic bearing fails, e.g. as the result of a power failure, the rotor shaft falls into a safety bearing and is retained thereby. A safety bearing therefore serves to retain the rotor shaft. The safety bearing temporarily takes over as the mounting of the rotor shaft until the rotor shaft is at a complete standstill. Safety bearings must firstly withstand the shock when the rotating rotor shaft falls into the safety bearing and, secondly, ensure safe running down of the rotor shaft in the safety bearing. For this purpose, the safety bearing has a slightly larger internal diameter compared with the rotor shaft diameter so that, during normal operation, that is, with the magnetic bearing active, the rotor shaft does not touch the safety bearing. Usually, the safety bearing is accommodated in the machine carcass in the region of the respective end of the rotor shaft.
Safety bearings must firstly withstand the shock when the rotating rotor shaft falls into the safety bearing and, secondly, ensure safe running down of the rotor shaft in the safety bearing. For this purpose, certain frictional characteristics and kinematic conditions must be met. Excessively high friction between the sliding or rolling components would lead within a very short time to severe heating and thus to a short service life of the safety bearing. This has the result that running down of the rotor shaft in the safety bearings without braking is not possible in most cases. Therefore, for the safe operation of machines in which the rotor shaft is mounted with magnetic bearings, braking devices must generally be provided for decelerating the rotor shaft.
If a magnetic bearing fails, the rotor shaft falls, as stated above, into the safety bearing. The danger exists herein that the rotor shaft performs a “backward whirl”, rolling along the inner surface of the safety bearing. By contrast to rotationally synchronous rotor motion wherein the rotor deflection takes place synchronously with the circulating imbalance excitation, in the case of a backward whirl, the rotor shaft performs the orbit in the reverse direction to the rotor shaft rotation with a very large amplitude. A portion that is rotationally synchronous and has a much smaller amplitude is overlaid, so that an elliptical orbit is produced.
The conditions for the occurrence of a backward whirl are manifold. The occurrence of backward whirl generally leads, due to the very large forces involved, to disruption or damage of the machine.
The use of roller bearings as safety bearings is known from the prior art. The outer ring of a roller bearing is connected to the bearing end plate. The inner diameter of the inner ring of the safety bearing is somewhat larger than the outer diameter of the rotor shaft. During a crash, the rotor shaft falls into the inner ring so that the inner ring and the rolling bodies are accelerated after a very short time and the rotor shaft runs down. A safety bearing based on a roller bearing is, firstly, unsuitable for large rotor weights and, secondly, the danger of backward whirl exists.
Furthermore, the use of dry sliding bearings as safety bearings is known. The rotor shaft falls directly into a fixed ring which comprises individual coated bearing shells, and runs down there. Given unfavorable frictional characteristics, the rotor shaft is able to enter into backward whirl.
Previously, attempts were made, using complex proofs based on calculations and experiments, to show that backward whirl does not occur in the aforementioned known safety bearings, taking account of all the known framework conditions. This type of procedure is time-consuming and costly.
It is an object of the invention to provide a safety bearing in which the probability of backward whirl occurring is greatly reduced compared with safety bearings from the prior art.
This aim is achieved with a safety bearing for retaining a rotor shaft of a machine wherein the safety bearing has a first carrier body rotating about a virtual geometrical central axis, and rolling bodies, the rolling bodies each having a region which is arranged between the central axis and the first carrier body, the rolling bodies each being rotatably connected to the first carrier body via a shaft.
Advantageous embodiments of the invention are disclosed in the dependent claims.
It has proved to be advantageous if the rolling bodies are configured as rollers. Rollers are a common configuration of the rolling bodies and are particularly easy and economical to manufacture.
It has also proved to be advantageous if the first carrier body is configured as a ring, since a ring has a form that is particularly mechanically stable.
It has also proved to be advantageous if the rolling bodies are arranged evenly distributed round the periphery of the first carrier body, since a backward whirl can then be particularly reliably prevented.
It has also proved to be advantageous if the rolling bodies are each rotatably connected via a shaft and at least one roller bearing to the first carrier body, since then a particularly low coefficient of friction is achieved.
It has also proved to be advantageous if the safety bearing has a second carrier body arranged round the first carrier body, elastic elements being arranged between the first carrier body and the second carrier body. By this means, the shock loading acting on the rolling bodies and the roller bearings in the event of a crash of the rotor shaft into the safety bearing is reduced.
It has also proved to be advantageous if the second carrier body is configured as a ring, since a ring has a mechanically particularly stable form.
It has also proved to be advantageous if the elastic elements are configured as spring damping elements or as damping elements. The configurations of the elastic elements given above are common configurations of the elastic elements.
It has also proved to be advantageous if the elastic elements are arranged, relative to the virtual geometrical central axis, radially aligned with the rolling bodies.
It has also provided to be advantageous if the elastic elements are arranged offset in a tangential direction from the rolling bodies. This provides a particularly simple overall arrangement.
It has also proved to be advantageous to configure a machine with safety bearings according to the invention. Here, the machine preferably has a magnetic bearing for operational mounting of the rotor shaft.
It has also proved to be advantageous to configure a machine having the safety bearing according to the invention. The machine preferably has a magnetic bearing for operational mounting of the rotor shaft. The machine can be configured, for example, as an electric motor or a generator or a compressor or a condenser or as a turbine. The machine can particularly be configured as a wind power generator.